Presently the most ubiquitous candidate for supplying solar energy is still the silicon-based module using a p-n junction. The p-n junction refers to the interface between two segments of silicon that have been doped with impurities, such as boron or phosphorus so that there is a preponderance of holes and electrons in these two segments, respectively. The majority charge carrier in the boron-doped segment is the hole and this segment is denoted as p-type. The majority charge carrier in the phosphorus-doped segment is the electron and this segment is denoted as n-type. As shown in FIG. 1A, upon light absorption (the arrow) an electron (black circle) is promoted from the valence band to the conduction band leaving a hole (white circle) in the valence band. These carriers are free to drift under the influence of the energetic gradient established by the p-n junction to their respective electrodes, as illustrated in FIG. 1B, for electricity production.
The environmental abundance of silicon coupled with accumulated knowledge on the operation of the device is what makes it commercially attractive. Furthermore, for every doubling of cumulative production there has been a 20% cost reduction over the past 30 years. Industry leaders such as Sunpower are selling modules that are 20% efficient and are achieving huge cost savings as they scale up production.
A common trend among solar panel manufacturers is to increase module efficiency in hopes that fewer solar panels will be required to generate a given amount of power. Reducing the total number of panels also helps reduce the balance of systems (BOS) cost, typically roughly $1.30/Watt for utility-scale power generation. Even for state-of-the-art silicon based cells sold by companies like Sunpower, the high BOS cost is not necessarily because of the module itself, but because of all the additional components that must be installed with it. The BOS includes the land on which the module is mounted, the mounting, monitoring systems, and labor. While the module alone may cost about $0.70/Watt, the BOS increases costs by roughly $1.30/Watt, making the entire installation nearly twice as expensive as the current $1/Watt threshold for market competitiveness. Even the installation of a free module would be prohibited because the BOS cost of $1.30/W is excessive.
Any high performing solar panel must absorb light, generate electric current, and conduct electric current very well. Silicon based solar panels clearly have many merits, but also have significant shortcomings. One of the most prominent limitations of these modules is thermalization after light absorption, which reduces the energy available to do electrical work. When an electron-hole pair is excited optically with energy greater than the band gap of silicon, 1.1 eV, the electron reaches a state higher than the conduction band edge, then relaxes back to the band edge by rapidly emitting thermal energy. In essence, any photon energy greater than 1.1 eV is thus wasted in this process of thermalization. For this reason, light energy is not utilized efficiently with any semiconductor.
Due to such inherently low efficiency, other approaches have been sought for converting solar energy into electricity. In one alternative approach an organic based solar cell has been provided (G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger. Science. 1995, 270, 1789.), a representative organic solar cell made from a polymer, poly(3-hexylthiophene), or P3HT, and phenyl-C61-butyric acid methyl ester, or PCBM. In a P3HT:PCBM solar cell, P3HT serves as an electron donor and hole conductor and the PCBM serves as an electron acceptor. The functionality of the donor-acceptor junction under light absorption strongly resembles that of the p-n junction (as illustrated in FIG. 2). When light (represented by the arrow in FIG. 2A) is absorbed, bound electron-hole pairs (the electron represented by the black circle, the hole by the white circle), or excitons, are generated primarily in P3HT as it is the more highly absorbing compared to PCBM. After this exciton is split at the donor-acceptor interface, the electron drifts to the cathode and the hole to the anode, as depicted in FIG. 2B to provide electricity.
This technology suffers in the phases of light absorption and in charge transport of photo-generated carriers (electrons and holes). The organic absorber tends to have a narrow absorption range so it does not harvest the full solar spectrum. Charge transport of photo-generated carriers is hampered because of the disorder of the material, whether it is a polymer or a small molecule. Charge transport in organic materials is typically poor in comparison to crystalline inorganic materials and, as a result, the electrons may recombine with holes before they reach the electrodes. Rather than providing electricity, these charges recombine and release light which is counter-productive. Thus, organic solar cells are limited both in terms of the amount of light absorbed and in the extraction of electricity.
Given these drawbacks of the prior art, a most viable strategy is one which aims to produce a module at low cost and reduce the BOS cost, and that requires an ultra-efficient module with the performance exceeding the Shockley-Queisser limit of 30%.